Conversion of hydrocarbons



Feb. 9, 1937. H. TROPSCH CONVERSION OF HYDROGARBONS Filed Dec. 15, 1933 FRACTIONATOR INVENTOR HANS T PSCH I TORNEY Patented Feb. 9, 1937 STATES PATENT OFFICE CONVERSION OF nYmtocARBoNs Hans Tropsch, Chicago, Ill., assignor to Universal Oil Products Company, Chicago, 111., a corporation of Delaware Application December'15, 1933. Serial No. 702,433

4 Claims.

This invention relates more particularly to the conversion of the propane-butane fractions of hydrocarbon gas mixtures such as, for example,

natural gas, gas mixtures produced incidental to 5 the primary distillation of crude petroleums and the fixed gases produced in cracking operations upon heavy hydrocarbon oils. The term propane-butane fraction is thus intended to include fractions containing the .corresponding olefins, (propylene and the butylenes) which will constitute a varying proportion of cracked gases. It

is also intended to include fractions produced from stabilizer units operating upon either straight run or cracked gasolines which have the approximate composition of propane-butane mixtures although the boiling range may be somewhat wider in these cases due to crude fractionation; e. g. may contain small percentages of lower boiling members of the group such as ethane and methane and some higher boiling members, such as pentane. Pentanes and amylenes representing five carbon atom hydrocarbons are not objectionable in charging stocks to be treated by the present invention, but it is preferred that methane present as they are distinctly detrimental as will be developed later in the description.

In a. more specific sense the invention is concerned with a pyrolytic process for emciently converting propane-butane fractions into commercial yields of good anti-knock gasoline, the limiting conditions of operation under which optimum conversion into desired; gasoline liquids is oba tained being a particular feature of the process. 5 The hydrocarbon fractions which serve as the charging material according to the present invention are too light for blending in gasoline above a certain limit. Also, the total quantities of such gases produced in petroleum refining operations, 40 including cracking, is very large and, from the standpoint of the refiner, they areuslially looked upon as a waste product since, from a fuel stand- I point, they are produced in excess of requirements for the heating of cracking processes and frequently for the total heating of all refinery distilling units, and in many cases low grade fuels are available.

A more profitable utilization of these propane butane fractions is, therefore, a matter ofsome importance and the present invention is a contribution to this field.

In one specific embodiment the present invention comprises heating hydrocarbon mixtures consisting essentially of compounds of 3 and 4 carbon atoms under selected elevated temperature and pressure conditions, passing the heated products through an enlarged reaction zone in which they are mainiained at substantially constant pressure 'and temperature conditions for a time necessary for the completion of desired con.-

and ethane be absent or only minor amounts be hydrocarbons is accomplished will be brought out in the following description of the sequence of 15 operating steps given in connection with a particular plant, hookup shown in the attached diagrammatic drawing in which interconnected 'ele-. ments are represented by conventional figures in side elevation not drawn to scale. '20

Referring to the. drawing hydrocarbon charg ing stocks of the general character previously designated (which will be hereinafter referred to V as propane-butane" mixtures), may be introduced to the plant by way of line i, containing control valve 2, and pumped by a pump 3 through a line 4 containing a control valve 5 into and through'a properly designedtubularheating element 6 positioned in a furnace setting I. During 1 passage of the gases through the heating 'ele- 30 ment and the subsequent reaction zone, the interrelated factors of temperature, pressure and time. are so controlled that about 50 to by weight of the original charge appears later as intermediate recycle stock. It has been found that-if 35 higher rates of conversion per pass occur not only is the ultimate yeld of liquefiable gasoline hydrocarbons reduced, but also carbon troubles Y are encountered in the heating element so' that dangerous overheating of the tubes may' take place unless-- frequent shutdown periods are observed. Furthermore, by proper control of the operating" conditions while maintaining a. restricted conversion per pass, the formation of .heayy tars may be practically eliminated so that these 45 ordinarily useless byproducts of cracking are not formed. If lowerrates of conversion per pass are employed it has been found that the equipment may be too large for economy. As a general average, when dealingwith propane-butane o mixtures ordinarily obtained from stabilizer units, the production of 60% recycle stock is an averagefigure for best overall yields.

' The temperature range employed at the exit of the heating element is from 450 to 575 (3., in 5 a majority of cases from; 525 to 550 0., and the pressure may vary from approximately 25 to atmospheres, though as a rule when dealing with the commoner propane-butane blends" 59 to 75 1 atmospheres produce optimum results in respect cc to yield of gasoline. In regard to temperature, it

may be noted that the lower ranges are prefer able when thegas treated, for example, a gas from an oil cracking plant, contains a relatively high percentage of olefins, say from 30 to.40%,

while the higher temperatures accomplish more desirable effects when the olefin content of the original gases is low or approximately from to 20% volume concentration. The time under which the reacting substances are maintained under these temperature and pressure conditions The heated products from the heating element 6 pass through transfer line 8, containing control valve 9, and enter a reaction zone it without substantial pressure reduction other than a slight differential drop due to fluid friction in the line. The reactions of polymerization which occur in this chamber are of an exothermic character and for best results the chamber may be cooled somewhat to maintain the optimum polymerizing temperature for the production of gasoline boiling range liquids rather than heavier polymers which are of too high boiling range for use in gasoline. The cooling may be broughtabout by controlled exposure of the chamber to atmospheric conditions or by placing indirect heat exchange coils in the chamber through which if desired the incoming charge may be pumped countercurrently:

The products from the reaction zone comprising fixed gases, gasoline and small amounts of heavier polymers is then conducted through line ll, containing control; valve 12, to a chamber 13 prb 'vided principally as a separator for any heavy tars which are settled and withdrawn through line ,containing control valve l5. Thezpressure is preferably reduced at valve l2 to some point within the range of 150 to 300 pounds per square inch, depending upon the relative amounts of the three types of products, to-wit, gases of two carbon atoms and less, intermediate'recycle stock consisting of compounds of 3 and i carbon atoms and gasoline hydrocarbons. Within this pressure range at ordinary temperatures, it is found that the intermediate recycle stock compounds are liquid either alone .or at least completely soluble in the condensed gasoline.-

The vaporous products from the tar separator pass through line l6, containing control valve II, to a iractionator 20 which is operated to finally separate compounds heavier than gasoline and of the character of intermediate petroleum dis tillates which are withdrawn through line l8, containing control valve l9, to any use for which they are suited, while the overhead vapors comprising the major portion or the admitted products pass through line 21, control valve 22 condenser 23, line 24 and valve 25 to receiver 26 which is preferably maintained under the same pressure as that in the preceding tar separator and iractionatcr, allowing for diflerential drops.

Under a pressure range of approximately 150 to 300 pounds the'gaseous products present in receiver 26 are found to consist principally of hydrogen, methane, ethane and ethylene, and as a. rule the separation is sufficiently sharp so that these gases may be. withdrawn more or less completely from the receiver through line' 2L-containing control .valve 28. The removal of the hydrogen process appears in the plant under description and theJow molecular weight parafllns at this point is of positive value while the loss of ethylene is of no consequence since this compound 'does not polymerize to any great extent to formgasoline under the preferred-conditions of operation.

The liquids in receiver 26 comprising essentially propane, propylene, butanes, butylenes, and liquid gasoline hydrocarbons is then taken by pump 3! through line 29, containing control valve 30 and subjected to treatment aimed at the segregation of 3 and 4 carbon atom recycle stock andthe desired gasoline product.

The discharge from pump 3| the mixture may be mildly heated during passage through'a heater coil 34 positioned in the furnace 35 and brought to a temperature of approximately 100 to 150 F., leaving the heating element through line 36, containing control valve 31, and entering a iractionator 0r stabilizer 38. By suitably utilizing refluxes in this fractionator the 3 and 4 carbon atom recycle stock may be sharply separated from the gasoline so that the vapor pressure of the final product is at the required point, usually in the neighborhood of ten pounds per square inch by-the Reid vapor pressure test. It is, of course, to be understood that any conventional stabilizer may be empl0yed.,

The ovehead products from the fractionator pass through line 46, control valves 41, condenser 48, line 49 and control valve 50 to receiver which acts as an accumulator for the recycle fractions. Any residue of fixed gases still present at this'point may be withdrawn through gas release line 52, containing control'valve 53, and utilized as fuel if desired,

Recycle stock accumulating in receiver 5| is returned to the primary heating element. For best results the combined feed blend of this stock with the raw feed should comprise not more than 35% of oleflns such as propylene and butylenes.

When conditions are properly regulated in the passing through .line 32 and control valve 33 therein is separated exceeds 35% this may be reduced by admixing propane-butane mixtures of a more parafllnic character such as would be obtained, for example, from stabilizer units operating upon straight run gasoline. The importance of this point resides in the fact that if the oleflns in the feed are too high there is excessive deposition of carbcn in the heating tubes and a corresponding loss in fixed gases of less than 3 carbon atoms under the optimum conditions of treatment. The

recycle stock is returned to the heating zone by way of line 54, control valve 55, recycle pump 56, line 51 and valve 58 to enter combined feed line 4.

Gasoline representing the final product of the as a bottom 'reflux from fractionator 38 from which it may be withdrawn through line 39, con- 1 taining control valve 60, to receiver 4| with any necessary intermediate cooling although means for effecting this are not shown in the drawing. The receiver is provided with a gas release line d2 containing control valve 63 and a liquid draw line 55, containing control valve 45, for the release of final product. i

As exemplifying the relationship obtaining betweentime, temperature and the percent of higher olefins in charging stock within a definite temperature interval the following data are introduced. In the-experiments furnishing the data, pressure was held constant at 710 pounds per square inch, the recycle stock was 60% of the charge and comprised approximately 25% of propylene and butylenes.

In computing the actual contact time, the following formula was used:

, Contact tirne (seconds)=i where: n

t is the time of the experiment in seconds,

P is the pressure in atmospheres, 7

Va is the volume of the apparatus (heated zone) in the same units asVv,

Vv is the volume which the charge would have were it completely gasified at 0 C. and 760 mm. pressure plus the volume of the products corrected to 0 C. and 760 mm. divided by two. The two volumes may be expressed in any convenient units so long as Va and Vv are in the same units,

T is the temperature in C.

The compressibility of the gases was ignored inthese calculations. It was assumed at these high temperatures that the perfect gas laws are valid.

The calculated contact time was obtained by using the following empirical formula:-

10g1ot=(7.1140.0105T-0.0112U)logiod whe e:

. of the charge was recovered as recycle stock,

then.d=40.

' TABLEI-.

Relation between percent of gas reacted, olefin content, temperature and'contact time A Percent de- Actual Calculated Percent Tempera- Percent contact contact anon higher ture C. d time time calculated oleflns 1 seconds seconds 333 37 500 23. 5 98 -10. 9 37 500 26. 5 +4.35 37 525 37. 5 70 -72 +2. 8 37 525 40. 4 75 78 3 37 .525 40. 6 74 78 4 28 525 35. 5 107 97 i 35 28 550 46. 7 67. 7 -53 21. 7 28 550 7 45. 2 66. 2 51 23- 0 24 525 -32. 8 I26 105 l6. 7 24 525 38.3 143' I 9.8 2 531 44. 2 9s :00 +4.2 24 537 34. 0 86 72 -16. 3 24. X 537 26. 0 67 52 -22. 2 24 55 37. 6 53. 5 49 v 8. 4' 24 550 34. 7 45 45 f 0 24 550 i 42. 5 I 57. 3 58 0 U=l0 fora. gas containing 10% high-- 7 '42), the liquid yield is higher.

Average deviation=4.5%. The following experimental examples are characteristic of results obtainable by the present process.

Example I.Using a charging stock whichcontains no olefins, having an average molecular weight of 60, gives a combined feed containing 16% higher olefins. This combined feed requires a heating time of 120 seconds at 537 C; (1000 F.). At 550 C. (1022 F.) only 75 seconds are necessary when using a pressure of 710 pounds per square inch in both cases. In this case a yield of 10 gallons per 1000 cubic feet of gas mixture was obtained, or 300 barrels of gasoline from 1000 barrels of liquefied charging stock. The liquid product contained 90% gasoline with an A. P. Lgravity of 61.5 60 F. and a blending octane number (motor method) of 93. The Eng- 'ler distillation of the gasoline fraction follows:

Percent bottoms 1.0

Example II..Using a -charging stock which contains 37% higher olefins, and has an average molecular weight ,of 51, gives a combined feed having 28% olefins. At 525 C. (977 F.) a time .jactor of 120 seconds is necessary while-at 537 Q.

a time of '72 seconds is necessary for 40% conversion at 710 pounds per square inch pressure. The yield in this case is about eight gallons of liquid product per 1000 cubic feet of stock charged. The product contains 90% gasoline which has an A. P. I. gravity of 64.0 and a blending octane number (motor method) of 93. The Engler distillation follows:

"I first example is 10 gallons per 1 000 cubic feet of gas reacted while in the second case the yield is I. B. P. F s9 E.-P. F 392 Percent over 97.0

Percent bottoms l. 1.0

Percent loss I 2.0

It will be noticed that the yield of liquid in the 8 gallons per 1000 cubic feet of gas reacted. In

the conditions described is zerb. With purepropane (molecular weight 44) the yield is-about two gallonspef' 1 000- cubic feet of gas reacted. If'on' the other hand the propane contains appreciableamounts of propylene (molecular weight Thus, the yield of liquid depends on the molecular weight of. the gas charged and the-amount of oleflns present,

since it'is fairly easy to form liquids from the oleflns already in the gas, while thjormation of 75 oleilns from the parafiins is accompanied by relatively large losses in the form of non-condensable gases, particularly-ii the paraflns have a.

1'5 limitations upon the scope of the'invention. 1

I claim as my invention: 1. A process for producing more valuable products from light hydrocarbon material consisting essentially of hydrocarbons. of 3 and 4 carbon 20 atoms, which comprises subjecting said material, in admixture with'recycle stock formed as hereinafter set forth; to pyrolytic conversion such as to convert a portion thereoNnto gasoline boiling hydrocarbons, controlling'the reaction conditions as of temperature, pressure and timein the con-' version step so that only from, 20% to 50% by weight of said material undergoes alteration into hydrocarbons or less than 3 and more than 4 car- 7 hon-atoms, separating from the eiiiuent of the conversion step. gaseous hydrocarbons of less than -3 carbon atoms and isolating the same from the process, further separating from said e'iiiuent the 3 and 4 carbon atom hydrocarbons, supplying these separated ,3 and 4 carbon atom hydrocaras bons tothe conversion step as said recycle stock. and recovering theh'ydrocarbons o! more than 4 J bon atoms from which the 3 and 4 carbon atom drocarbons have been separated.

2. A process for producing more valuable products from light hydrocarbon material co'nsi'sting essentially oi' hydrocarbons or 3 and 4 carbon-atoms, which comprises subjecting said material, in admixture with recycle stock formed as hereinafter set forth, to 'pyrolytic conversion at a temperature of from 450' c. to 575 0. and

under a pressure of from 25 to 100 atmospheres for a time period such that only from 20% to 50% by weightoi' saidimaterial undergoes alteration into hydrocarbons of less than 3 and more than 4 a carbon atoms, separating from the emuentot the conversion step gaseous hydrocarbons of less than 3-carbon atoms and isolating th e sa'm e from the I process, further separating from said eiiluent the 3 and 4 carbon atom hydrocarbons, supplying these separated 3 and 4 carbon atom hydrocarbons to the conversion step as said recycle stock, and recovering the hydrocarbons or more than 4 carbon atoms from which the 3 and 4 carbon atom hydrocarbons have been separated.

3. A process for producing morevaluable products from the propane-butane Iractions separated from hydrocarbon gas mixtures, which comprises '10 subjecting the propane-butane traction, in admixture with recycle stock'formed as hereinafter" set forth, to pyrolytic conversion such as to produce gasoline boiling hydrocarbons, separating from the eiiiuent of the conversion step an interi5 mediate fraction consisting essentially of the 3 and 4 carbon; atom hydrocarbons of the eiiiuent, controlling the reaction conditions =0! tempera ture, pressure and time in the conversion step so that said intermediate fraction contains an amount of 3 and 4 carbon atom hydrocarbons corresponding to from 50% to80% by weight oi said propane-butane fraction and so that-saidintermediate fraction contains not more than oletlns, supplying said intermediate frac- 25 tion to the conversion step as said recycle stock, and isolating the remaining portion-oi said ei- 1 fluent from the process.

4. A process which comprisessubjecting a com- V bined'feed, formed as hereinafter set" 'iorthand consisting essentially 0K3 and 4 carbon atom hydrocarbons, to pyrolytic conversion'suchias to convert a portion thereof into gasoline'boilinl hydrocarbons. separating from the eiiiuent otthe' conversion step an intermediate traction con-s5,

'sisting essentially of the 3" and 4 atom hydrocarbons oi the eiiiuent, controlling the. reaction conditions of temperature, pressure and-time in the conversion step so that said intermediate fraction. contains not-more than 25% oleiinic hydrocarbons of '3 an d4 carbon atoms, combining said intermediate fraction .with the propan butane fraction of a hydrocarbon gas .mixture and formingjrom' these .fractions a mixture con taining lessthan oleflns andbet'w'een' and by weightoi' the intermediate fraction,

supplying the last-named; mixture to "the con version step as said combined reed. L

the remaining portion of said efliuen't, the process. 

